Late Aptian to late Albian sea-level fluctuations constrained by geochemical and biological evidence (Nahr Umr formation, Oman)

The Aptian to Albian interval of the vast Arabian Platform was studied in n orthern Oman (Nahr Umr Formation) for its paleoceanographic and sea level r ecord. The sections indicate seven transgressive-regressive cycles overprin ted by small-scale sea level fluctuations. The peak of regression lead to e mergence and exposure of the sea floor and overprinted marine hardground su rfaces. The subaerial overprint of marine hardgrounds is difficult to recognize in the field. However, four independent lines of evidence document emergence. These are: (1) Pronounced negative excursions in carbon and partly in oxyge n isotope composition beneath hardbottoms. They are attributed to the influ x: of light, soil-derived carbon and light oxygen from meteoric water. (2) Sparite-filled rudists beneath hardbottoms contain primary brackish-water f luid inclusions of very low salinity interpreted as cement precipitation in the phreatic marine/meteoric mixing zone or "pollution" of meteoric freshw ater by residual salt. (3) Symbiont-bearing and thus light-dependent orbito linids dis play small, conical morphotypes in the shallow, well illuminated water prior to exposure and hardbottom formation, Large, flat morphotypes are characteristic of the transgressive shales above those surfaces and doc ument deeper, less illuminated waters. (4) The presence of mycorrhiza (Micr ocodium). The exposure surfaces overlie facies incomplete shallowing-upward successio ns or subtidal sediments, but unequivocal evidence for erosion before or du ring emergence is lacking, Correlation of transgressive-regressive events i n Oman with other sea-level curves suggests the rec ord of spasmodic region al tectonism combined with a strong eustatic component.